Numbers 4:45
These be those that were numbered of the families of the sons of Merari, whom Moses and Aaron numbered according to the word of the LORD by the hand of Moses. These be those that were numbered of the families of the sons of Merari, whom Moses and Aaron numbered according to the word of the LORD by the hand of Moses.
Numbers 4 is the record of the Levites' service assignments, giving God's instructions for transporting the sanctuary. This verse belongs to that record as the entry for the numbering of the families of the sons of Merari, whom Moses and Aaron numbered according to the word of the Lord given by the hand of Moses. The surrounding chapter shows the Lord assigning sanctuary work to the Levites by specific duties, with men from thirty to fifty years old numbered and given service, including both priests and other Levites.
Those transport instructions required the Kohathites to carry the ark and the sacred vessels on their shoulders without touching them. The numbering shows how careful God was in counting His people, with each tribe numbered by itself under careful census inspectors, and shows His careful division of labor among the Levites, exemplifying the ordered organization of Israel for the work assigned to them. Apart from showing that carefulness, the verse is a detail of no special interest.
The prescribed order explains why placing the ark on a cart and Uzzah's touching it violated God's due order.